Acoustic resonators can be used to implement signal processing functions in various electronic applications. For example, some cellular phones and other communication devices use acoustic resonators to implement frequency filters for transmitted and/or received signals. Several different types of acoustic resonators can be used according to different applications, with examples including bulk acoustic wave (BAW) resonators such as thin film bulk acoustic resonators (FBARs), coupled resonator filters (CRFs), stacked bulk acoustic resonators (SBARs), double bulk acoustic resonators (DBARs), and solidly mounted resonators (SMRs). An FBAR, for example, includes a piezoelectric layer between a first (bottom) electrode and a second (top) electrode over a cavity. BAW resonators may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, FBARs may be used for electrical filters and voltage transformers.
An acoustic resonator typically comprises a layer of piezoelectric material sandwiched between two plate electrodes in a structure referred to as an acoustic stack. Where an input electrical signal is applied between the electrodes, reciprocal or inverse piezoelectric effect causes the acoustic stack to mechanically expand or contract depending on the polarization of the piezoelectric material. As the input electrical signal varies over time, expansion and contraction of the acoustic stack produces acoustic waves that propagate through the acoustic resonator in various directions and are converted into an output electrical signal by the piezoelectric effect. Some of the acoustic waves achieve resonance across the acoustic stack, with the resonant frequency being determined by factors such as the materials, dimensions, and operating conditions of the acoustic stack. These and other mechanical characteristics of the acoustic resonator determine its frequency response.
In general, an acoustic resonator comprises different lateral regions that may be subject to different types of resonances, or resonance modes. These lateral regions can be characterized, very broadly, as a main membrane region and peripheral regions, where the main membrane region is defined, roughly, by an overlap between the two plate electrodes and the piezoelectric material, and the peripheral regions are defined as areas outside the main membrane region. Two peripheral regions, in particular, are defined as a region located between the edge of the main membrane region and edge of the air-cavity, and a region of an overlap of at least one plate electrode and the piezoelectric material with the substrate. The main membrane region is subject to electrically excited modes generated by the electric field between the two plate electrodes, and both the main membrane and the peripheral regions are subject to certain derivative modes generated by scattering of energy in the electrically excited modes. The electrically excited modes comprise, for instance, a piston mode formed by longitudinal acoustic waves with boundaries at the edges of the main membrane region. The derivative modes comprise, for instance, lateral modes formed by lateral acoustic waves excited at the edges of the main membrane region and the peripheral regions.
The lateral modes facilitate continuity of appropriate mechanical particle velocities and stresses between the main membrane region and the peripheral regions. They can either propagate freely (so called propagating modes) or exponentially decay (so called evanescent and complex modes) from the point of excitation. They can be excited both by lateral structural discontinuities (e.g., an interface between regions of different thicknesses in the main membrane region, or an edge of a top or bottom electrode) or by electric field discontinuities (e.g., an edge of a top electrode where the electric field is terminated abruptly).
The lateral modes generally have a deleterious impact on the performance of an acoustic resonator. Accordingly, some acoustic resonators include ancillary structural features designed to suppress, inhibit, or mitigate the lateral modes. For example, a collar may be formed by a dielectric material outside the boundary of the main membrane region to allow a smooth decay of evanescent modes emanating from the boundary and improve confinement of mechanical motion to the main membrane region. In another example, a frame may be formed by a conductive or dielectric material within the boundary of the main membrane region to minimize scattering of electrically excited piston mode at top electrode edges and improve confinement of mechanical motion to the main membrane region.
The conventional implementation of these ancillary structural features has a number of potential shortcomings. For instance, depending on their specific design, they may be a source of additional scattering of the piston mode which may outweigh their benefits. Additionally, they may require the presence of certain additional materials that can deleteriously redistribute the acoustic energy in the acoustic stack, such as relatively soft planarization layers. Also, some design choices may produce only modest performance improvements while significantly driving up cost. Moreover, the formation of ancillary structural features may degrade structural stability or interfere with the formation of overlying layers.
In addition, conventional FBARs rely on strong confinement of electrically excited piston mode. Strong confinement is provided by the edges of the top and bottom electrodes, as well as ancillary structural features, such as air-bridges and conventional outside frames. While the apparent advantage of strong confinement is that it prevents strong electrical excitation of mechanical motion at the edge of the top electrode, it also provides significant acoustic discontinuities, leading to scattering of energy out of the desired piston mode into undesired extensional, shear, flexural and dilatational modes of the whole structure. Accordingly, in view of these and other shortcomings of conventional acoustic resonator structures, there is a general need for improved acoustic resonator designs.